Modern imagesetters and platesetters utilize optical scanners to write or record images for subsequent reproduction or to read a prerecorded image at a predefined resolution rate. Such scanners may write or record images on or read prerecorded images from various media including photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings or erasable imaging materials, an aluminum or other metal base printing plate, or other type media. The medium is typically mounted on an imaging surface which may be planar or curved and then scanned with an optical beam.
The primary components of modern imagesetting and platesetting systems include an image processor to generate and/or edit an image, a raster image processor (RIP) for converting data signals from the image processor into signals which can be understood by a controller which controls the imaging by the imagesetter or platesetter. The imagesetter or platesetter itself typically includes an imaging head such as a scan assembly which is often disposed and movable within a drum cylinder in which the recording or recorded medium is mounted. The controller, in accordance with the signals from the RIP and its own programmed instructions, generates signals to control the optical scanning so as to write images on or read images from the medium mounted within the drum cylinder by scanning one or more optical beams over the inside circumference of the drum cylinder while the cylinder itself remains fixed. A typical scan assembly of a cylindrical drum type imager system may include a spin mirror or other optical device to direct the light beam over the inside circumference of the drum cylinder, as will be well understood by one skilled in the art.
In order to provide quality imaging, it imperative that the medium to be imaged be properly positioned on the support surface of the imaging system. Conventionally, registration pins, punches or notches are utilized for this purpose. In this regard, the applicable registration devices are fixedly mounted along the medium support surface such that, when the medium is loaded into the imaging system it is positioned against the registration devices. Although suitable for use in imaging systems which are intended to image only media of one width, such fixed registration devices are less than ideal for use in imaging systems which are intended to image media of varying widths.
Typically, it is assumed that the medium being loaded onto the support surface prior to imaging is properly positioned by forcing the medium to be imaged against the registration devices. However, if the medium is being loaded in a skewed disposition, the medium may contact less than all the registration devices and therefore be skewed on the support surface at the time of imaging.
To confirm proper contact between the loaded medium and the registration device, it has been proposed to provide electric detection circuits which are closed when a metallic medium contacts a metallic registration device. However, such circuits can only confirm whether or not the medium is properly positioned and are useless when a non-metallic medium is being imaged